Use rust-toolchain.toml to specify target and component requirements.

This way, the requirements are automatically available to all users,
not just specifically the GitHub Actions CI.

A caveat is that `rustup override set` ignores the file entirely, so
in order to change toolchains we have to patch the file instead of
using the override command. Still seems better overall.

.github/workflows/ci.yml

@@ -92,12 +92,13 @@ jobs:
 
     - run: df -h .
 
-    - name: Install Rust toolchain
-      # Install exactly what we need: compiler, Cargo, clippy, rustfmt
+    - name: Set Rust toolchain
+      # The rust-toolchain.toml file specifies the targets and components we need,
+      # but we may want to override the toolchain.
+      if: ${{ matrix.toolchain != 'stable' }}
       run: |
-          rustup toolchain install "${{ matrix.toolchain }}" --profile=minimal --component=clippy --component=rustfmt
-          rustup target add --toolchain="${{ matrix.toolchain }}" wasm32-unknown-unknown
-          rustup override set "${{ matrix.toolchain }}"
+          sed -i "s/stable/${{ matrix.toolchain }}/" rust-toolchain.toml
+          rustup show  # triggers installation of selected toolchain
 
     - name: Install nightly for -Z direct-minimal-versions
       if: ${{ matrix.depversions == 'minimal' }}

.github/workflows/unstable-ci.yml

@@ -63,16 +63,12 @@ jobs:
       with:
         ref: unstable-rust
 
-    - name: Install Rust toolchain
-      # Install exactly what we need: compiler, Cargo, clippy, rustfmt
+    - name: Set Rust toolchain
+      # The rust-toolchain.toml file specifies the targets and components we need.
+      # Therefore, CI only needs to set the toolchain override.
       run: |
-          rustup toolchain install "${{ matrix.toolchain }}" --profile=minimal --component=clippy --component=rustfmt
-          rustup target add --toolchain="${{ matrix.toolchain }}" wasm32-unknown-unknown
           rustup override set "${{ matrix.toolchain }}"
-
-    - name: Install nightly too
-      if: ${{ matrix.depversions == 'minimal' }}
-      run: rustup toolchain install nightly --profile=minimal 
+          rustup show  # triggers installation of selected toolchain
 
     - name: Install native libraries
       if: ${{ runner.os == 'Linux' }}

rust-toolchain.toml

@@ -0,0 +1,5 @@
+[toolchain]
+channel = "stable"
+targets = ["wasm32-unknown-unknown"]
+profile = "minimal"
+components = ["clippy", "rustfmt"]